Each of the four plant genera has one
or more native, solitary, specialist bees that collect
pollen only from them. In turn, these bees provide
effective pollination services for their floral hosts
(Figs. 1, 3, 8, 12, 14). Female bees that specialize
in collecting pollen from one or a few closely related
species of flowering plants are called oligoleges, the
process is oligolecty (oligo = few; legere
= to gather). These specialist (oligolectic) bees
require pollen from their floral hosts to provide
protein for development of their offspring. Life
cycles of these bees are closely synchronized with
those of their pollen host flowers (see below).

Generalist bees and other flower
visiting insects also visit most of these flowers
(Figs. 9, 10, 11, 15) and may potentially pollinate
them. Do the flowers require the specialist bees for
their reproduction? Certainly in the case of Blennosperma
nanum, Dr. Joan Leong demonstrated in her PhD
thesis, that seed set was significantly lowered in
experimental plant arrays placed at a created vernal
pool site where only generalist pollinators were
available as compared to those experimental plant
arrays placed at Jepson Prairie Reserve where there is
a complete guild of flower visitors including the
specialist (oligolectic) bee, Andrena (Diandrena)
blennospermatis. Her experiments show that the
specialist bee is important to the reproduction of the
plant.

Female bees of the genus Andrena that
specialize on pollen of vernal pool flowers are on
the wing only in early spring when their host plants
are in flower. The life cycle of the bee is closely
tuned to that of its host plant bloom period. The
bees emerge at or just before their host flowers
initiate bloom. Male bees emerge shortly before the
females. Mating occurs as soon as females emerge.
Females initiate construction of their brood nests
immediately after mating.

The female bees construct their nest in the soil
of the upland areas surrounding the pools. The
presence of such bee nests is revealed by ant
mound-like clumps of excavated soil (tumuli) at the
surface of of the soil. These are best seen in the
early morning while the soil excavated and pushed to
the surface by the female during the night is still
damp. As these circular piles of earth dry, they may
be eroded by wind and rain. The tumuli (Fig. 16) may be
clustered in areas of suitable soil, or scattered
over the area. They may be associated with bare
soil, but often become hidden beneath growing
vegetation such as the expanding basal leaf rosettes
of filaree. There is no indication of social
structure in the biology of these bees. A closed
tumulus (Fig. 17) indicates the female is in her burrow
constructing it. An open tumulus (Fig. 18) indicates
the female bee is afield foraging for pollen and/or
nectar as provisions for her offspring.

The basic nest architecture (Fig. 19) consists of a
vertical shaft penetrating the soil for a few inches
before becoming a lateral tunnel ending in a single
chamber (brood cell). A newly constructed brood
cell (Fig. 20) has a polished look from the water-proof
lining. This waxy lining is secreted by the female
and painted over the inner surface of the brood
cell.

When brood cell construction is complete, the female
bee forages (Fig. 21) for pollen on her preferred host
flowers. She grooms pollen from her body and packs
it into specialized brushes of hair (scopae) on her
hind legs for transport back to her nest. As the dry
pollen food provisions (Fig. 22) accumulate in the
brood cell, the female adds nectar, moistening the
mass to dough-like consistency. When the pollen
ball (Fig. 23) is complete, she lays an egg on top. She
then constructs a brood cell cap (Fig. 24) closing off
the entrance to chamber with spirally arranged soil
particles.

After closing off the brood cell the mother bee
has no further contact with her offspring. Thus, she
is a solitary bee in contrast to the social honey
bee. She then digs another lateral tunnel, pushing
the soil into the tunnel leading to the most
recently completed brood cell. In the photo showing
the nest architecture (Fig. 19) a second, completed,
brood cell with pollen ball and egg is visible to
the left. The tunnel that connected this cell to the
vertical shaft was filled and no longer visible. If
one could look through the soil from above, the nest
would look like a series of spokes radiating out
from the vertical entrance shaft, each spoke ending
in a single brood cell.

The newly hatched larva (Fig. 25) begins feeding on
the provisions. After consuming all the provisions,
the post-feeding larva (Fig. 26) defecates and remains
quiescent in its brood cell throughout the remainder
of the spring and well into summer. In about
September, it transforms to a pupa (Fig. 27) which is
the transition stage between the grub-like larva and
the adult bee. The adult remains quiescent in its
brood cell until the following spring when it
emerges to continue its life cycle in association
with the flowering of its pollen host plant.

NB: The majority of the life cycle of these bees
is spent in the brood cell. Overwintering as an
adult rather than a post-feeding larva is an
adaptation to synchronize these bees with the early
spring bloom of their pollen host plants. In
contrast to most female insects that pump out
hundreds of eggs, these solitary bees may lay only
20 to 30 eggs in their lifetimes. Most of their
effort goes into providing a secure haven with a
mass of food provisions adequate for their
development. This maternal investment and year long,
single generation life cycle means that populations
of these bees recover very slowly from any
catastrophic losses.

Figure 16:
Nest
tumuli of Andrena on the surface of the
ground

Figure 17:
A closed nest tumulus with the female bee inside her
burrow

Figure 18:
An open nest tumulus indicates that the female bee
is outside foraging

Figure 19:
Nest architecture - a vertical shaft and a lateral
tunnel ending in a single brood cell . A second,
completed brood cell with pollen ball and egg is
visible to the left.

Figure 22:
Food provisions - early stage of formation of pollen
ball in a brood cell

Figure 23:
Pollen ball with bee egg on top

Figure 24:
Brood cell cap (closure) inner view

Figure 25:
Newly hatched bee larva on top of the pollen ball
provision

Figure 26:
Post-feeding larva in its brood cell

Figure 27:
Pupa in its brood cell

About the Contributors

Robbin Thorp is Professor Emeritus of Entomology.
Robbin initiated research on native bees and flowers
of the vernal pool ecosystem shortly after joining
the faculty at University of California, Davis in
1964. He continues his studies following his
retirement in 1994. He also retains appointment as
Chair of the Jepson Prairie Reserve Advisory
Committee at UC Davis.

Dennis Briggs was a Staff Research Assistant for
Professor Thorp's pollination research from 1967 to
1990. Dennis is responsible for most of the close-up
photographs of bees and flowers in this
contribution.